1. Field of the Invention
This invention relates to a process that is useful for polymerizing or copolymerizing propylene and more particularly concerns a process for producing a homopolymer or copolymer or propylene having increased stiffness and broadened molecular weight distribution.
2. Discussion of the Prior Art
Many homopolymers and copolymers of propylene have certain properties that are unsatisfactory for specific applications. For example, the stiffness or rigidity of certain homopolymers and copolymers of propylene, such as polypropylene and ethylene-propylene copolymers, is lower than the rigidity of polystyrene or ABS resin, and this fact has caused a serious restriction in broadening its application. In particular, biaxially stretched polypropylene films have inferior stiffness to cellophane and polyester films although they are packaging materials having excellent optical properties (such as transparency, luster, etc.) and moisture resistance. As a result, polypropylenes are not suitable for use in automatic packaging, especially overlap packaging and twist packaging, leading to a great limitation on their use. Moreover, even in cases where the polypropylene films can be made much thinner from the standpoint of moisture resistance and other properties, it is inevitably necessary to increase their film thickness to obtain stiffness. This is not only uneconomical but is also an obstacle to the miniaturization of electrical components such as dry condensers wherein a polypropylene film is used.
For example, if the rigidity of a polymer or copolymer of propylene is improved, it is possible to reduce the thickness of the resulting molded product formed from it. This is not only effective for the conservation of resources, but also the cooling rate at the time of molding can be increased; hence it is possible to make the molding rate per unit time faster and thereby improve the productivity in molding and processing.
Although many polymerization and copolymerization processes and catalyst systems have been described, it is advantageous to tailor a process and catalyst system to obtain a specific set of properties of a resulting polymer or copolymer product. For example, in certain applications a product with a broader molecular weight distribution is desirable. Such a product has a lower melt viscosity at high shear rates than a product with a narrower molecular weight distribution. Many polymer or copolymer fabrication processes which operate with high shear rates, such as injection molding, oriented film, and thermobonded fibers, would benefit with a lower viscosity product by improving throughput rates and reducing energy costs. Products with higher stiffness, as measured by flexural modulus, are important for injection molded, extruded, and film products since the fabricated parts can be down gauged so that less material would be needed to maintain product properties. Also important is maintaining high activity and low atactic levels such as measured by hexane soluble and extractable materials formed during polymerization or copolymerization. Thus, it is highly desirable to develop polypropylene having increased rigidity or stiffness and broadened molecular weight distribution.
For example, Chiba et al., U.S. Pat. No. 4,499,247 disclose a high-rigidity and high-melt-viscoelasticity polypropylene for sheets to be post-processed and for blow molding, which polypropylene is produced by subjecting propylene to a multi-stage polymerization into polymer portions of two sections in the presence of a specified Ziegler Natta catalyst. The catalyst is prepared by reacting or organoaluminum compound (I) or a reaction product (VI) of an organoaluminum compound (I) with an electron donor (A), with titanium tetrachloride (C); reacting the resulting solid product (II) with an electron donor (A) and an electron acceptor (B); and combining the resulting solid product (III), a titanium trichloride composition, with an organoaluminum compound (IV) and an aromatic carboxylic acid ester (V), so as to give a molar ratio of the aromatic carboxylic acid ester (V) to the solid product (III) of 0.1 to 10. The patent states that if a catalyst containing some other titanium trichloride composition instead of the aforesaid solid product (III), the desired beneficial results are not obtained. The relationship between the intrinsic viscosities of the polymer portions of the respective sections is regulated within a specified range and also the amount ratio of the polymer portions of the respective sections is regulated. The two polymer portions differ in their molecular weights, and this difference is produced by varying the concentration of hydrogen in the gas phase in the first stage from the concentration of hydrogen in the gas phase in the second stage. The aromatic carboxylic acid ester is employed as an external modifier (as described hereinbelow) in the polymerization in order to elevate the isotacticity of the resulting polypropylene. The polymerization can be carried out in the slurry or gas phase. Applications of polymers formed by a polymerization process employing essentially the same catalyst are disclosed in Fujishita et al., U.S. Pat. No. 4,560,734.
Chiba et al., U.S. Pat. No. 4,500,682, disclose a polypropylene having a superior post-processability and blow moldability, which polypropylene is obtained by polymerizing propylene in multiple stages using a catalyst comprising a titanium trichloride composition, an organoaluminmum compound and a molecular weight modifier, the resulting final polymer comprising a higher molecular weight portion and a lower molecular weight portion, and the final polymer consisting of 40 to 60 weight percent of polypropylene portion corresponding to the higher molecular weight portion and 60 to 40 weight percent of a polypropylene portion corresponding to the lower molecular weight portion. The gas-phase hydrogen concentration is adjusted from one stage to the next stage of polymerization in order to effect the production of polypropylene portions having different molecular weights in the different stages. The polymerization can be carried out in the slurry, bulk or gas phase.
Chiba et al., U.S. Pat. No. 4,550,144, disclose a propylene-ethylene block copolymer from which molded products having high rigidity and superior high impact properties can be prepared, which copolymer is obtained by (i) polymerizing propylene in 70 to 95 weight percent based on the total polymerized amount, using a catalyst obtained by reacting an organoaluminum compound (I) or a reaction product of (I) with an electron donor (A), with titanium tetrachloride, further reacting the resulting solid product (II) with (A) and an electron acceptor (B), and combining the resulting solid product (III) with an organoaluminum compound (IV) and an aromatic carboxylic acid ester (V), the molar ratio of (V) to (III) being 0.1 to 10.0, and then (ii) polymerizing ethylene or ethylene and propylene in 30 to 5 weight percent based on the total polymerized amount, in one or more stages, using the same aforesaid catalyst, the ethylene content being 3 to 20 weight percent based on the total polymerized amount. In specific examples of two-stage copolymerizations, the hydrogen concentration in the gas phase differs from one stage to the next stage. The copolymerization can be carried out in the slurry, bulk or gas phase. The patent states that if a catalyst containing titanium tetrachloride supported on a carrier such as magnesium chloride is employed instead of the aforesaid catalyst in the method disclosed therein, the described beneficial results are not obtained. Applications of the resulting copolymers are disclosed in Asakuno et al., U.S. Pat. No. 4,638,030.
Chiba et al., U.S. Pat. No. 4,582,878, disclose a high-rigidity, whitening-resistant ethylene-propylene copolymer, which is obtained by copolymerizing propylene with ethylene in three successive stages wherein in the respective first, second and third stages, a copolymer fraction having a specified ethylene content is formed in a specified amount based on the total polymerization amount, in the presence of a catalyst obtained by reacting an organoaluminum compound (L) or a reaction product thereof with an electron donor (E) with titanium tetrachloride; reacting the resulting solid product (I) with an electron donor (E) and an electron acceptor; and combining the resulting solid product (II) with an organoaluminum compound (L) and an aromatic carboxylic acid ester (R), the molar ratio of (R) to (II) being in the range of 0.1 to 10, and in the presence of hydrogen. The hydrogen concentration of the gas phase differs from one stage to the next stage of polymerization. The copolymerization can be carried out in the slurry, bulk or gas phase. The patent states that if a catalyst containing titanium tetrachloride supported on a carrier such as magnesium chloride is employed instead of the aforesaid catalyst in the method disclosed therein, the desired beneficial results are not obtained.
The use of other catalysts or catalyst components--namely, solid, transition metal-based catalyst components for the polymerization or copolymerization of alpha-olefins, including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide-based catalyst components--than those described as being useful for the process disclosed in the aforesaid U.S. Pat. Nos. 4,499,247; 4,500,682; 4,550,144; and 4,582,878 is well known in the art. Such hydrocarbon-insoluble, magnesium-containing, titanium-containing catalyst components are described in Hoppin et al., U.S. Pat. No. 4,829,038, which is incorporated in its entirety herein by reference. Also known is incorporating an electron donor compound into the titanium-containing component as an internal modifier. An olefin polymerization system typically comprises a titanium-containing compound, an alkylaluminum compound and an electron donor external modifier compound. The electron donor external modifier used in combination with the alkyl aluminum compound and solid titanium-containing compound is distinct from the electron donor which may be incorporated as an internal modifier within the titanium-containing compound. Many classes of electron donors have been disclosed for possible use as electron donor external modifiers used during polymerization.
One class of such electron donor compounds is organosilanes. For example in U.S. Pat. No. 4,540,679, organosilanes, especially aromatic silanes, are described. Use of organosilanes as cocatalyst modifiers also is described in Published U.K. Application 2,111,066 and U.S. Pat. Nos. 4,442,276, 4,472,524, 4,478,660, and 4,522,930. Other aliphatic and aromatic silanes used in polymerization catalyst are described in U.S. Pat. Nos. 4,420,594, 4,525,555 and 4,565,798.
Hoppin et al., copending, U.S. patent application Ser. No. 410,663, filed Sep. 21, 1989, disclose specific branched C.sub.3 -C.sub.10 alkyl-t-butoxydimethoxysilanes modifiers which not only are used in supported catalysts to provide high yield and low atactic products, but which also produce a polymer with a broader molecular weight distribution than produced using the organosilane compound selected from the group consisting of diisobutyldimethoxysilane diisopropyldimethoxysilane, di-t-butyldimethoxysilane and t-butyl-trimethoxysilane, and mixtures thereof, as described in Hoppin et al., U.S. Pat. No. 4,829,038, which as indicated hereinabove, in its entirety is specifically incorporated by reference herein.
Arzoumanidis et al., U.S. Pat. No. 4,866,022, discloses a method for forming a particularly advantageous alpha-olefin polymerization or copolymerization catalyst or catalyst component that is formed by a process that involves a specific sequence of specific individual process steps such that the resulting catalyst or catalyst component has exceptionally high activity and stereospecificity combined with very good morphology. A solid hydrocarbon-insoluble, alpha-olefin polymerization or copolymerization catalyst or catalyst component with superior activity, stereospecificity and morphology characteristics is disclosed as comprising the product formed by 1) forming a solution of a magnesium-containing species from a magnesium hydrocarbyl carbonate or magnesium carboxylate; 2) precipitating solid particles from such magnesium-containing solution by treatment with a transition metal halide and an organosilane; 3) reprecipitating such solid particles from a mixture containing a cyclic ether; and 4) treating the reprecipitated particles with a transition metal compound and an electron donor. This patent also discloses organosilanes that are useful as reagents in precipitating a solid from a soluble magnesium species and that have the formula R.sub.n SiR.sup.1.sub.4-n where n is 0 to 4, R is hydrogen or an alkyl, alkoxy, haloalkyl or aryl radical containing one to about ten carbon atoms or a halosilyl radical or haloalkylsilyl radical containing one to about eight carbon atoms, and R.sup.1 is OR or a halogen. The patent also discloses that aliphatic or aromatic silanes are advantageously employed as electron donor external modifiers and that preferred aliphatic silanes include isobutyltrimethoxysilane, diisobutyldimethoxysilane, diisopropyldimethoxysilane, di-t-butyldimethoxysilane, and t-butyltrimethoxysilane.
Arzoumanidis et al., U.S. Pat. No. 4,540,679, disclose a process for the preparation of an aforesaid magnesium hydrocarbyl carbonate by reacting a suspension of a magnesium alcoholate in an alcohol with carbon dioxide and reacting the magnesium hydrocarbyl carbonate with a transition metal component. Arzoumanidis et al., U.S. Pat. No. 4,612,299 disclose a process for the preparation of an aforesaid magnesium carboxylate by reacting a solution of a hydrocarbyl magnesium compound with carbon dioxide to precipitate a magnesium carboxylate and reacting the magnesium carboxylate with a transition metal component.